This invention relates to a high molecular weight aliphatic polyester having a novel copolymer structure and a method of preparing same.
Synthetic polymers such as polyolefins and aromatic polyesters are now used in a large amount as raw materials indispensable to daily life. These polymers, which are not decomposable in natural environment, causes environmental problems as an increase of consumption thereof.
In this circumstance, biodegradable plastics are now being developed. Among biodegradable polymers, aliphatic polyesters are attractive. In particular, much attention has been paid on polybutylene succinate produced from butane diol and succinic acid or a derivative thereof, because of their high mechanical strengths and melting point. (JP-A-H05-70566, JP-A-H09-12691, JP-A-H09-13259, etc.)
Since polybutylene succinate by itself does not necessarily provide practically sufficient mechanical strengths and processability, studies are being made to improve the properties thereof by copolymerization with various polyvalent alcohols or hydroxyacids. (JP-A-H09-31176, JP-A-H09-40762)
With these proposals, however, the mechanical strengths, especially, breaking strain, are still not fully improved.
It is an object of the present invention to provide a novel biodegradable high molecular weight aliphatic polyester which has good mechanical strengths, especially breaking strain, and good processability and to provide a method which can produce biodegradable high molecular weight aliphatic polyester in a industrially advantageous manner.
According to the present invention there is provided a biodegradable high molecular weight aliphatic polyester comprising an ester section A represented by the following formula (1):
(xe2x80x94COxe2x80x94R1xe2x80x94COxe2x80x94Oxe2x80x94R2xe2x80x94Oxe2x80x94)pxe2x80x83xe2x80x83(1)
wherein R1 represents a divalent aliphatic group having 1-12 carbon atoms, R2 represents a divalent aliphatic group having 2-12 carbon atoms and p is a molar fraction of the polyester fraction of the formula (1) contained in the polyester,
and an ester section B represented by the following formula (2): 
wherein R1 represents a divalent aliphatic group having 1-12 carbon atoms, R3 represents an aliphatic group having 1-18 carbon atoms and r is a molar fraction of the polyester fraction of the formula (2) contained in the polyester,
the molar fraction r of said ester fraction B being in the range of 0.001-0.10.
The present invention also provides a biodegradable high molecular weight aliphatic polyester comprising a product obtained by condensation of an aliphatic dicarboxylic acid diester of the following formula (3):
R4Oxe2x80x94COxe2x80x94R1xe2x80x94COxe2x80x94OR5xe2x80x83xe2x80x83(3)
wherein R1 represents a divalent aliphatic group having 1-12 carbon atoms and R4 and R5 each represent an alkyl group having 1-4 carbon atoms,
with an aliphatic glycol of the following formula (4)
HOxe2x80x94R2xe2x80x94OHxe2x80x83xe2x80x83(4)
wherein R2 represents a divalent aliphatic group having 2-12 carbon atoms,
and with 3-alkoxy-1,2-propane diol of the following formula (5): 
wherein R3 represents an aliphatic group having 1-18 carbon atoms,
the amount of the 3-alkoxy-1,2-propane diol being 0.001-0.10 mole per mole of said aliphatic dicarboxylic acid diester.
The present invention further provides a method of preparing a biodegradable high molecular weight aliphatic polyester, comprising reacting an aliphatic dicarboxylic diester of the following formula (3):
R4Oxe2x80x94COxe2x80x94R1xe2x80x94COxe2x80x94OR5xe2x80x83xe2x80x83(3)
wherein R1 represents a divalent aliphatic group having 1-12 carbon atoms and R4 and R5 each represent an alkyl group having 1-4 carbon atoms,
with an aliphatic glycol of the following formula (4)
HOxe2x80x94R2xe2x80x94OHxe2x80x83xe2x80x83(4)
wherein R2 represents a divalent aliphatic group having 2-12 carbon atoms,
and with 3-alkoxy-1,2-propane diol of the following formula (5): 
wherein R3 represents an aliphatic group having 1-18 carbon atoms,
the amount of the 3-alkoxy-1,2-propane diol being 0.001-0.10 mole per mole of said aliphatic dicarboxylic acid diester.
Other objects, features and advantages of the present invention will become apparent from the detailed description of the preferred embodiments of the invention to follow.
The biodegradable high molecular weight aliphatic polyester according to the present invention comprises an ester section A represented by the above formula (1) and an ester section B represented by the above formula (2).
In the above formula (1) representing the ester section A, R1 represents a linear or cyclic divalent aliphatic group having 1-12 carbon atoms, preferably 2-6 carbon atoms. Such a divalent aliphatic group may be an alkylene, such as methylene, ethylene, propylene, butylene, hexylene, octylene, dodecylene, cyclohexylene and cyclohesanedimethylene.
The symbol R2 represents a linear or cyclic divalent aliphatic group having 2-12 carbon atoms, preferably 2-6 carbon atoms. Such a divalent aliphatic group may be an alkylene group such as methylene, ethylene, propylene, butylene, hexylene, octylene, dodecylene, cyclohexylene and cyclohexanedimethylene.
In the formula (2), R3 represents a linear or cyclic aliphatic group and R1 has the same meaning as above. The aliphatic group R3 has 1-18 carbon atoms. Examples of such aliphatic groups include alkyl groups and alkenyl groups, such as methyl, ethyl, propyl, butyl, hexyl, octyl, nonyl, decyl, dodecyl, octadecyl, lauryl, stearyl, behenyl, dodecenyl, cyclohexylene and cyclohexanedimethylene.
The amount of the ester section B in the polyester (molar fraction r) is 0.001-0.10, preferably 0.002-0.03. When the amount of the second ester section B is excessively great, the degree of polymerization of the polymer obtained is not large so that the polymer becomes brittle. On the other hand, too small an amount of the ester section B causes problems that the breaking strain of the polyester is small and the physical properties are similar to those of the homopolymer.
The high molecular weight aliphatic polyester according to the present invention may be produced by various methods. One preferred method includes condensing a divalent aliphatic dicarboxylic acid diester of the following formula (3) with an aliphatic glycol of the formula (4) and with a 3-alkoxy-1,2-propane diol of the formula (5).
The aliphatic dicarboxylic acid diester of the formula (3) may be a diester (such as dimethyl ester, diethyl ester, dipropyl ester or dibutyl ester) of an aliphatic dicarboxylic acid such as succinic acid, adipic acid, suberic acid, sebacic acid or dodecanedicarboxylic acid.
The aliphatic glycol of the formula (4) may be ethylene glycol, propylene glycol, 1,4-butanediol, 1,6-hexanediol, 1,4-cyclohexanedimethanol, polyethylene glycol or polypropylene glycol. The aliphatic glycol is generally used in an amount of 0.90-1.10 moles, preferably 0.95-1.05 moles, per mole of the aliphatic dicarboxylic acid diester.
The 3-alkoxy-1,2-propane diol of the formula (5) may be obtained by, for example, reacting glycerin with an alcohol. The alcohol may be a saturated or unsaturated alcohol having an aliphatic group having 1-18 carbon atoms. Illustrative of suitable alcohols are methanol, ethanol, propanol, butanol, octanol, nonyl alcohol, lauryl alcohol, pentadecyl alcohol, cetyl alcohol, heptadecyl alcohol, stearyl alcohol, allyl alcohol, crotyl alcohol, cyclopentanol and cyclohexanol. The 3-alkoxy-1,2-propane diol is used in an amount sufficient to give a polymer having a molecular weight of at least 10,000 and is, generally, 0.001-0.10 moles, preferably 0.005-0.05 mole, per mole of the aliphatic dicarboxylic acid diester. An amount of the 3-alkoxy-1,2-propane diol more than the above range is not preferable because the degree of the polymerization of the polymer (polycondensation product) is not large and the polymer becomes brittle.
The condensation reaction is preferably carried out in the presence of a conventional ester exchange catalyst.
The reaction is generally performed at a temperature so that the hydroxyl compounds derived from the aliphatic dicarboxylic acid diester and produced as by-products can exist as a gas in the reaction system. When R4 OH and R5 OH by-products are each methanol, for example, the reaction is generally performed at 100-300xc2x0 C., preferably 120-250xc2x0 C. The reaction pressure is generally under a reduced pressure, an ambient pressure or a slightly pressurized condition (0.5 kg/cm2G or less) . An ambient pressure or a reduced pressure is preferably adopted. It is preferred that the reaction be performed using a reactor equipped with a distillation tower (reaction distillation tower) for removing the hydroxyl compounds produced as by-products from the reactor as quickly as possible.
The reaction is preferably carried out in two steps of a preliminary condensation step (first step) and a molecular weight increasing step (second step). In the preliminary condensation step, low molecular weight condensation products having a terminus to which the aliphatic glycol has been bonded are produced. The condensation product has a number average molecular weight of 500-10,000, preferably 1,000-5,000 The molecular weight can be suitably controlled by the reaction conditions and reaction time. The reaction conditions are such that the hydroxyl compounds produced as by-products can exist as gas.
In the molecular weight increasing step, the low molecular weight condensation products are further condensed while eliminating the aliphatic glycol bonded at their termini to form a high molecular weight condensation product. This step can yield a condensation product having a number average molecular weight of at least 10,000. The reaction conditions may be such that the aliphatic glycol produced as by-products can exist as a gas. The molecular weight increasing step may be carried out using the same reactor as used in the preliminary condensation step or a polymerization apparatus having good stirring efficiency. When the same reactor is used, the reaction conditions are changed, after completion of the preliminary condensation step, for example, by increasing the reaction temperature and reducing the reaction pressure, to perform the condensation of the preliminary condensation products.
The high molecular weight polyester of the present invention has a number average molecular weight of at least 10,000, preferably at least 30,000. The upper limit of the number average molecular weight is generally about 1,000,000.
The high molecular weight polyester of the present invention has a copolymer structure derived from the monoacylated glycerin of the formula (2). As a result of the addition of a small amount of the 3-alkoxy-1,2-propane diol, the mechanical strengths (especially breaking strain) and the processability of the polyester are improved. Further, the polyester has biodegradability.